1. Field of the Invention
The present invention relates to a voltage adjusting apparatus, especially a voltage adjusting apparatus used for a hybrid vehicle (HEV) to detect a voltage value of an on-vehicle battery.
2. Description of the Related Art
At present time, a hybrid vehicle driven by usage in parallel of an engine and a motor for traveling has become popular. An electric circuit of the hybrid vehicle includes a low-voltage system for driving the engine and a high-voltage system for driving the motor for traveling. The low-voltage system has an on-vehicle low-voltage battery of 12 volts. The high-voltage system has an on-vehicle high-voltage battery formed by a battery pack in which a plurality of unit cells is connected in series. The high-voltage battery is given a high voltage by connecting in series the plurality of unit cells, which is a secondary battery such as a nickel-metal hydride battery or a lithium cell.
The high-voltage battery after repeating charge and discharge has a dispersion of a voltage value between both electrodes of each unit cell, that is, a dispersion of a State of Charge (SOC). For securing durability of respective unit cells and safety, when the unit cell having the highest SOC (or the highest voltage value between both electrodes) is charged to reach the predetermined upper limit SOC (or upper limit voltage value between both electrodes), it has to be prohibited to charge the high-voltage battery. And, when the unit cell having the lowest SOC (or the lowest voltage value between both electrodes) is discharged to reach the predetermined lower limit SOC (or lower limit voltage value between both electrodes), it has to be prohibited to discharge the high-voltage battery. Therefore, when there is the dispersion of SOC between respective unit cells, an actual available discharging capacity is reduced. In case of an application for HEV, assist drive and regeneration, which assists an engine by consuming energy from the battery on climbing and regenerates energy back to the battery on descending, are not performed. Thereby, running performance and fuel consumption are degenerated.
For a solution, an equalizing apparatus, which adjusts (equalizes) SOC of each of a plurality of unit cells in the high-voltage battery, and thereby improves the dispersion of SOC between each unit cell, is proposed. The equalizing apparatus performs equalizing action by power supply from the low-voltage battery during an ignition switch is OFF, that is during the engine is OFF. Since the equalizing action is performed during the engine is OFF, the low-voltage battery is only discharged without charging. Therefore, when the low-voltage battery is over-discharged by supplying electric power to the equalizing apparatus during the engine is OFF, the low-voltage battery loses energy enough to start the engine.
For solving above problems, another equalizing apparatus, which includes a control unit activated by power supply from the low-voltage battery and controls to stop equalizing action for preventing over-discharging of the low-voltage battery when it is judged by a self-diagnostics of the control unit that a battery voltage of the low-voltage battery is reduced less than a predetermined value, is proposed (in Patent document 1: Japan Patent Application 1 No. 2003-189490).
As another technical idea of self-diagnostics, an electronic control apparatus with self-diagnostics for an air-bag system of the vehicle, which arranges two test circuits for fail-safe monitoring a malfunction of each test circuit to each other when the test circuit self-diagnoses with respect to required functions for controlling the system, and transmits test results and monitoring results through a duplex communication line to a processor for judging, is proposed (Japan Patent Application 2 No. 2005-063054).